Re: issues with doubles and -fno-schedule-insns2

[Matthew J Fletcher <mfletcher at serck-controls dot co dot uk>]

On Friday 26 Nov 2004 2:27 pm, Nathan Sidwell wrote:
> Matthew J Fletcher wrote:
> > Hi
> >
> > i have a C code snipit that causes problems with gcc 3.3.4 and 3.4.1, i
> > have not tried the v4 snapshots, in all cases are am hosted and targeting
> > on i386.
> >
> >
> > am i doing something wrong ?
>
> yes. You are breaking the type based aliasing rules.  Your second example
> 'happens' to work, but don't rely on it doing so for later releases
> or other slightly different uses.

thats good to know,..

however i forgot to mention that i am targeting (march=i486) for my embedded 
sc520, dont know if its relivant to the following. i dont understand what 
'schedule-insns2' is doing, but it seems to be doing the wrong thing, again i 
could be very wrong.

int main()
{
        unsigned int Value[2] = { 0x7FF00000, 0x7FF00000 };
        double *ValueD = (double *)&Value[0];

        //(*ValueD) == nan(0x07ff00000)
        printf("Value = %g\n",(*ValueD));

        return 0;
}

 // gcc 3.3.4 -O3 -fno-schedule-insns2 -march=486
 // produces correct result
 08048370 <main>:
 8048370:       55                      push   %ebp
 8048371:       89 e5                   mov    %esp,%ebp
 8048373:       83 ec 08                sub    $0x8,%esp
 8048376:       83 e4 f0                and    $0xfffffff0,%esp
 8048379:       c7 45 f8 00 00 f0 7f    movl   $0x7ff00000,0xfffffff8(%ebp)
 8048380:       c7 45 fc 00 00 f0 7f    movl   $0x7ff00000,0xfffffffc(%ebp)
 8048387:       51                      push   %ecx
 8048388:       8b 45 fc                mov    0xfffffffc(%ebp),%eax
 804838b:       50                      push   %eax
 804838c:       8b 55 f8                mov    0xfffffff8(%ebp),%edx
 804838f:       52                      push   %edx
 8048390:       68 84 84 04 08          push   $0x8048484
 8048395:       e8 ee fe ff ff          call   8048288 <_init+0x38>
 
 // gcc 3.3.4 -O3 -march=486
 // produces wrong result
 08048370 <main>:
 8048370:       55                      push   %ebp
 8048371:       89 e5                   mov    %esp,%ebp
 8048373:       83 ec 08                sub    $0x8,%esp
 8048376:       83 e4 f0                and    $0xfffffff0,%esp
 8048379:       8b 45 fc                mov    0xfffffffc(%ebp),%eax
 804837c:       51                      push   %ecx
 804837d:       50                      push   %eax
 804837e:       8b 55 f8                mov    0xfffffff8(%ebp),%edx
 8048381:       52                      push   %edx
 8048382:       68 84 84 04 08          push   $0x8048484
 8048387:       c7 45 f8 00 00 f0 7f    movl   $0x7ff00000,0xfffffff8(%ebp)
 804838e:       c7 45 fc 00 00 f0 7f    movl   $0x7ff00000,0xfffffffc(%ebp)
 8048395:       e8 ee fe ff ff          call   8048288 <_init+0x38>

 note, movl's after push's which, i think just pushed crap onto the stack.


 so i tried a diffrent example.

int main()
{
        unsigned long long Value = 0x7FF000007FF00000;
        double *ValueD = (double *)&Value;

        //(*ValueD) == nan(0x07ff00000)
        printf("Value = %g\n",(*ValueD));

        return 0;
}

// gcc 3.3.4 -O3 -march=486
// produces correct result

 8048370:       55                      push   %ebp
 8048371:       89 e5                   mov    %esp,%ebp
 8048373:       83 ec 08                sub    $0x8,%esp
 8048376:       83 e4 f0                and    $0xfffffff0,%esp
 8048379:       50                      push   %eax
 804837a:       68 00 00 f0 7f          push   $0x7ff00000
 804837f:       68 00 00 f0 7f          push   $0x7ff00000
 8048384:       68 84 84 04 08          push   $0x8048484
 8048389:       e8 fa fe ff ff          call   8048288 <_init+0x38>

which works,..

any pointers to 'type based aliasing rules' ? docs for simple folk ?

regards
---
Matthew J Fletcher
Embedded Software
Serck Controls Ltd
---
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